Distributed services provided by Content Delivery Networks (CDNs) and Clouds are of increasing popularity. They are designed to offer powerful solutions for developing new services meeting users expectations while improving the Quality-of-Experience (QoE). It results of a growing traffic demand on network resources. In such a context, traffic optimization becomes a key challenge for both networks operators and CDN/Cloud services providers. This paper presents a cross-layer framework promoting the collaboration between the network and the service layers through a mutual exchange of information for an effective traffic optimization. CDN/Cloud service providers can expose high-level information about their needs and constraints, which helps network operators to guide them in their application level traffic management, to best use the network resources. Our evaluation comparing the performance of our cross-layer optimization framework with other approaches demonstrates that noticeable gains can be achieved on the network utilization and the perceived quality of services.